Just use an open back shirt that isn't your comp shirt. One that is a little too big so you can put it on yourself. Make sure you lift in a power rack and set the pins so you can hit your chest but if you miss, the bar can come forward and nott crush your throat
Others don't think it's going to be productive training:
make sure to use safety chains just below your lowest point of your stroke to save your ass if you can't lock it out.
if you wear a DD it won't stay in position when you lift off to yourself. If you wear a closed back shirt you won't be able to get it off. Not to mention its hard to handle weight 50-200# over your raw max without a good liftoff.
If you can get a "useful" bench shirt on w/o a partner then it isn't worth wearing. Might just as well wear a t shirt.